I'm just about to step through my code to replace some unnecessary #includes with forward declaration to speed up compile time. However, I'm looking for some advice or guideline on how far I should take this. Consider this class:

Obviously, this interface doesn't need to know anything about eigther StageGroup or DrawCall, so forward declaration would obviously do. However I need to include the header files whenever I want to use the interface. Furthermore, if I implement a class using this interface, I can still use the (existing) forward declaration, but at some point I will need to include the header files again, probably in the application code. Whats preferable in this case? When should I use forward declaration or when should I still include header files in such a use case?

This is arguable and really depends on a lot of factors. I personally take it as far as possible because it is easier to add includes later than it is to try and clean them up after they have been in use for a while. Saying that though, I also try to match my namespace/classes to obvious include paths. For instance, if I forward reference IMesh a lot then I need to use it in an implementation somewhere, what would the include path be? For my usual method of working: "acl/gfx/IMesh.hpp" would be the path and file name following the pattern I use. I started using this pattern fairly strictly after a couple SDK's began using it and I saw how nice and clean it kept things in most cases. Yes, you end up with a lot more includes in the cpp's themselves, but the headers remain significantly cleaner and more understandable in the long run. As long as the headers are easy to simply "know" what to include, there is no real hassle, if it is hunt and peck, that's bad news.

Of course, others disagree and I won't argue. My only comment is in significant code bases, I have seen no need to use precompiled headers in many cases as they didn't gain much speed when the includes are kept as clean as possible.

Always explicitly #include the headers you require - if you need header A and B then explicitly include both, don't rely on the fact that that header A happens to include header B. (Exception to this is if header A is specifically a grouping/convenience/pre-compiled header of headers).

An intermediate is having a header that contains the forward declarations for another header. So if you have foo.h then foo_fwd.h that has the forward declarations and relevant typedefs for the classes that appear in foo.h. At least in my opinion, this really only has value for template classes, but I've seen people use it for non template classes that are in nested namespaces as well.

I also try to match my namespace/classes to obvious include paths. For instance, if I forward reference IMesh a lot then I need to use it in an implementation somewhere, what would the include path be? For my usual method of working: "acl/gfx/IMesh.hpp" would be the path and file name following the pattern I use. I started using this pattern fairly strictly after a couple SDK's began using it and I saw how nice and clean it kept things in most cases.

Yep, this is just the way I handle it. Well, currently the main directory is namend "editor, any chance of renaming it without have to import every file into the MSVC solution explorer manually?

Thanks to all of you for the advice, so I'm going for full forward declaration.

You should always prefer forward declaration as it will also avoid circular dependencies which are never fun to resolve.

Yeah, had quite a time with those. Didn't think I can un-include the header files when I forward declare though, but now I do. Thanks again, all of you.

For compilation-time-with-header-files blues, you really should be looking at setting up pre-compiled headers instead. Longer term that's going to be of far more benefit to you.

It appears that the gentleman thought C++ was extremely difficult and he was overjoyed that the machine was absorbing it; he understood that good C++ is difficult but the best C++ is well-nigh unintelligible.

For compilation-time-with-header-files blues, you really should be looking at setting up pre-compiled headers instead. Longer term that's going to be of far more benefit to you.

Interesting, reading about them all the time but havn't cared about them until now. I'm going to have a read how to set them up in Visual Studio 2012. So you suggest using precompiled headers instead of forward declaration, or as an addition?

Precompiled headers work best for headers that rarely change. So you generally just put system headers, standard library headers and third party library headers in a precompiled header file. You use things like forward declarations to make your own headers that change more frequently compile more quickly.

Okay, I see, since I'm not using that much third-party libs (just directx and some standard-libaries, of course), I'm not going through the trouble right now.

But just another question, since I'm designing my "engine" with use by other users in mind, and seperate from the game I'm developing, how would I go about that? Obviously most third-party libs I know at least offer the convenience of being able to do things like:

Now with the new forward declaration method, I'd need to also include "Gui/ToolButton.h". Thats a little inconvenient, which doesn't matter for the lib itself, but should I really expect the user to include everything in advance of his compile time, or is there some prefered solution for this?

One option is to create helper headers for each subsystem of your library. In your case, you could have a gui.h header that just includes all the other headers that have items that live in the gui namespace.